Miso is fermented soybean.
The most common dashi soup stocks for miso soup are made of niboshi (dried baby sardines), kombu (dried kelp), katsuobushi (thin shavings of dried and smoked bonito, aka skipjack tuna), or hoshi-shiitake (dried shiitake).
The kombu can also be used in combination with katsuobushi or hoshi-shiitake. The kelp and/or shiitake dashi serve as a vegetarian soup stock.
Outside Japan, American or European style miso soup is sometimes made by dissolving miso in a Western vegetable stock. The stock might include ingredients such as negi, carrot, potato and daikon radish. In some versions of the dish chicken stock, Western-style fish stock, and other non-dashi bases can even be used, but there is some debate over whether or not miso soups made using these non-traditional bases count as true misoshiru. Christian Japanese refugees who came to the Philippines during the Edo period brought along miso soup.